A seal seam is the line where layers of film are joined by heat and pressure. The base and edges of a bag are closed with these seams. The seam is made with heat, not with a needle; even so, the industry refers to it as a “seam”.
The seal seam is one of the most heavily stressed areas of a bag. With insufficient sealing, the layers do not fully fuse and the seam opens under load. With excessive heat, the film right next to the seam thins and a tear starts there. There are different seam types, such as the bottom seal, side seal and star seal. Seam quality is checked in quality control visually and with simple loading or tensile tests.